Metastasis is a major cause of death in cancer patients and involves multistep processes including detachment of cancer cells from a primary tumor, invasion of surrounding tissue, spreading through circulation, re-invasion and proliferation in distant organs. KiSS-1 is a human metastasis suppressor gene that has been shown to inhibit metastases of human breast carcinomas and melanomas. We and others have demonstrated that the KiSS1 peptide is a ligand for a specific G-protein coupled receptor. G-protein coupled receptors (GPCRs) represent important specific targets for a number of human diseases, ranging from the control of blood pressure, hormonal disorders, neurological diseases, to tumorigenesis and metastasis. Activation of KiSSl G-protein coupled receptor (KiSS1 GPCR) leads to the inhibition of cell proliferation, migration, and invasion in our preliminary in vitro experiments. However, the roles of KiSSl together with its cognate GPCR in tumor progression and metastasis are poorly understood and the signaling mechanisms of KiSSl and its GPCR have not been elucidated. Therefore, the long-term objectives of our research are to understand the functions of KiSSl, KiSSl GPCR, and its signaling pathways in tumor progression and metastasis; and to determine whether KiSSl and its GPCR can be used as a drug candidate and molecular target, respectively, in the treatment of human cancer metastasis. In this application, our 3 specific aims are: 1) Determine the roles of KiSSl and KiSSl GPCR in the complex processes of breast cancer metastasis including cell migration, invasion, tumorigenesis, and metastasis using in vitro cell culture systems, syngeneic and experimental breast tumor metastasis models; 2) Understand the in vivo function of KiSSl and its GPCR in breast cancer progression and metastasis using transgenic mouse models that overexpress KiSSl and its GPCR in mammary gland epithelial cells; 3) Determine the signaling mechanism of KiSSl and KiSSl GPCR mediated functions in tumor metastasis, especially the Rho family GTPases and their signaling pathways. Results obtained from these studies will not only elucidate the biological function of KiSSl and its GPCR in breast tumor progression and metastasis, but also reveal how KiSSl and its GPCR directly regulate signaling pathways in cell migration and invasion. The long-term benefit of this research is the great potential of KiSSl and its GPCR as an important drug candidate and molecular target for anti-cancer drug discovery and tumor metastasis treatment.